6.3.1 - The fused plug
In many situations there is a need for
socket outlets to be closely spaced so that they are available
to feed appliances and equipment without the need to use
long and potentially dangerous leads. For example, the domestic
kitchen worktop should be provided with ample sockets to
feed the many appliances (deep fat fryer, kettle, sandwich
toaster, carving knife, toaster, microwave oven, coffee
maker, and so on) which are likely to be used. Similarly,
in the living room we need to supply television sets, video
recorders, stereo players, table lamps, room heaters, etc.
In this case, more outlets will he needed to allow for occasional
rearrangement of furniture, which may well obstruct access
to some outlets.
If each one of these socket outlets were
wired back to the mains position or to a local distribution
board, large numbers of circuits and cables would be necessary,
with consequent high cost. The alternative is the provision
of fewer sockets with the penalties of longer leads and
possibly the use of multi-outlet adaptors. Because the ideal
situation will have closely-spaced outlets, there is virtually
no chance of more than a small proportion of them being
in use at the same time, so generous allowance can be made
for diversity. Thus, cables and protective devices can safely
he smaller in size than would he needed if it were assumed
that all outlets were simultaneously fully loaded.
Thus a ring circuit protected by a 30 A
or 32 A device may well feed twenty socket outlets. It follows
that judgement must be used to make as certain as possible
that the total loading will not exceed the protective device
rating, or its failure and inconvenience will result. Two
basic steps will normally ensure that a ring circuit is
not overloaded.
1. - Do not feed heavy and steady
loads (the domestic immersion heater is the most obvious
example) from the ring circuit, but make special provision
for them on separate circuits.
2.
- Make sure that the ring circuit does not feed too
great an area. This is usually ensured by limiting a single
ring circuit to sockets within an area not greater than
one hundred square metres.
Fig 6.3 Plug and socket
to BS 1363
We have already indicated that a 30 A or
32 A fuse or circuit breaker is likely to protect a large
number of outlets. If this were the only method of protection,
there could be a dangerous situation if, for example, a
flexible cord with a rating of, say, 5 A developed a fault
between cores.
{Figure 3.13} shows that a 30 A semi-enclosed fuse will
take 5 s to operate when carrying a current of almost 90
A, so the damage to the cord would be extreme. Because of
this a further fuse is introduced to protect the appliance
and its cord. The fuse is inside the BS 1363 plug, and is
rated at 13 A or 3 A, although many other ratings up to
12 A, which are not recognised in the BSS, are available.
A plug to BS 1363 without a fuse is not
available. The circuit protection in the distribution board
or consumer's unit covers the circuit wiring, whilst the
fuse in the plug protects the appliance and its cord as
shown in {Fig 6.4}. In this way, each appliance can be protected
by a suitable fuse, for example, a 3 A fuse for a table
lamp or a 13 A fuse for a 3 kW fan heater.
Whilst
the installer of the wiring is seldom concerned with the
flexible cords of appliances connected to it, he must still
offer guidance to users. This will include fitting 3 A fuses
in plugs feeding low rated appliances, and the use of flexible
cords which are of sufficient cross-section and are as short
as possible in the circumstances concerned. Generally, 0.5
mm² cords should be the smallest size connected to plugs
fed by 30 A or 32 A ring circuits. Where the cord length
must he 10 m or greater, the minimum size should be 0.75
mm² and rubber-insulated cords are preferred to those that
are PVC insulated.
This type of outlet is not intended for
use at high ambient temperatures. A common complaint is
the overheating of a fused plug and socket mounted in an
airing cupboard to feed an immersion beater; as mentioned
above, it is not good practice to connect such a load to
a ring circuit, and if unavoidable, final connection should
be through a fused spur outlet.
Fig 6.4 Principle of
appliance protection by plug fuse
The British fused plug system
is probably the biggest stumbling block to the introduction
of a common plug for the whole of Europe (the 'europlug').
The proposed plug is a reversible two~pin type, so would
not comply with the Regulations in terms of correct polarity.
If we were to adopt it, every plug would need adjacent fuse
protection, or would need to be rewired back to its own
protective device. In either case, the cost would be very
high.
Ring circuits fed from systems where no
earth terminal is provided by the Electricity Supply Company
(TT systems) must be protected by an RCD rated at 30 mA,
In all installations, a socket intended to feed equipment
outdoors must be individually protected by a 30 mA RCD.
Where a socket is mounted on a vertical
wall, its height above the floor level or
the working surface level level must be
such that mechanical damage is unlikely. A minimum mounting
height of 150 mm is recommended.
